Screw machine

ABSTRACT

A screw machine ( 10 ) has a rotor housing ( 12 ) defining overlapping bores ( 13, 15 ). Female rotor ( 14 ) is located in bore ( 13 ) and male rotor ( 16 ) is located in bore ( 15 ). The end faces ( 24, 26 ) of the female and male rotors, respectively, have depressions ( 44, 46, 54, 56, 64 ) formed in their surface whereby the interface area with the facing surface ( 51 ) of the outlet casing ( 53 ) is reduced.

[0001] This application claims benefit of U.S. provisional application Ser. No. 60/172,767, filed on Dec. 20, 1999.

BACKGROUND OF THE INVENTION

[0002] In a conventional screw machine, a male rotor and a female rotor, disposed in respective parallel overlapping bores defined within a rotor housing, coact to trap and compress volumes of gas. While such two rotor configurations are the most common design, screw machines are also known in the art having three, or more, rotors housed in respective overlapping bores so as to coact in pairs. Paired male and female rotors differ in their lobe profiles and in the number of lobes and flutes. For example, the female rotor may have six lobes separated by six flutes, while the conjugate male rotor may have five lobes separated by five flutes. Accordingly, each possible combination of lobe and flute coaction between the rotors occurs on a cyclic basis.

[0003] The rotors of a typical screw machine are mounted in bearings at each end so as to provide both radial and axial restraint. Nevertheless, in conventional practice, a certain amount of clearance in the axial direction must be provided between the end face of the rotors and the facing surface of the housing. The need to provide an end running clearance is primarily the result of thermal growth of the rotors as a result of gas being heated in the compression process. Maintaining the desired end running clearance at an amount sufficient to ensure that contact does not occur between the end face of the rotors and the facing surface of the housing is important to reliable operation of the screw machine. Additionally, during operation, the pressure gradient in the fluid being compressed normally acts on the rotors in an axial direction tending to force the rotors toward the suction end of the screw machine, thereby tending to increase the end running clearance.

[0004] If the end running clearance is too large, excessive circumferential and radial leakage of compressed fluid may occur through the running clearance at the discharge end of the screw machine thereby significantly decreasing the overall efficiency of the screw machine. In conventional oil-flooded screw machines, it is customary to supply oil to the interface zone defined by the end running clearance between the rotor end faces and the housing end plate as a means of providing a fluid seal to reduce gas leakage through the interface zone. However, as the end running clearance is reduced, efficiency losses due to viscous friction forces in the oil between the rotor end faces and the housing end plate tend to increase.

[0005] As noted previously, in operation the rotors grow in the axial direction toward the end casing at the discharge end of the housing due to thermal growth resulting from the fluid being heated in the compression process. This thermal growth of the rotors tends to reduce the end-running clearance. However, during operation the aforenoted axial pressure gradient tends to push the rotors in an axial direction towards the suction end of the screw machine, thereby tending to increase the end running clearance.

[0006] Therefore, in conventional oil-flooded screw machines, it is customary to maintain a substantial amount of end running clearance to minimize friction losses and, in the extreme, to prevent failure from rotor seizure. Such seizure may result by the thermal growth of the rotor due to the compression process augmented by thermal growth from heat generated by the aforementioned friction forces. As the end running clearance decreases, these viscous friction forces increase and may generate sufficient additional heat to cause further thermal growth, leading to further reduction in the end-running clearance.

[0007] As noted previously, the penalty for maintaining a large end running clearance is a consequent increase in leakage of compressed fluid. In order to maintain a large end running clearance in conventional oil-flooded screw compressors, it is known to add material to the end face of the rotors to provide a physical barrier to circumferential gas leakage. For example, elongated bar strips have been welded to rotor end faces so as to extend radially along the centerline of the lobes or lands of the rotors thereby extending across and bridging a substantial portion of the end-running clearance.

SUMMARY OF THE INVENTION

[0008] It is an object of this invention to improve operating efficiency in a screw machine.

[0009] It is another object of this invention to reduce rotor end leakage in a screw machine.

[0010] It is a further object of this invention to reduce frictional losses, without increasing leakage, between the rotor end faces and the housing end plate in a screw machine. In the screw machine of the present invention, the interface surface area between the rotor end faces and the end casing is reduced by reason of a reduction in the surface area of at least either the rotor end faces or the facing surface of the outlet casing. In one embodiment of the present invention, discrete, non-interconnected, relatively large depressions are formed in the rotor end faces. In another embodiment, the rotor end faces may have a textured surface providing a multiplicity of interconnected, relatively small depressions. In another embodiment of the present invention, the surface of the outlet casing facing the rotor end faces is textured or otherwise provided with depressions.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] For a fuller understanding of the present invention, reference should now be made to the following detailed description of various embodiments thereof and to the accompanying drawings wherein:

[0012]FIG. 1 is a transverse section through a screw machine;

[0013]FIG. 2 is a partially sectioned view of the screw machine of FIG. 1;

[0014]FIG. 3 is an enlarged view of a portion of the discharge end of the screw machine of FIG. 1;

[0015]FIG. 4 is an end view of the rotors taken along line 4-4 of FIG. 3 showing one embodiment of the end faces of the rotors;

[0016]FIG. 5 is an end view of the rotors showing an alternate embodiment of the end faces of the rotors: and

[0017]FIG. 6 is an end view of the rotors showing a further alternate embodiment of the end faces of the rotors.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] Referring now to FIG. 1, there is depicted a screw machine 10, such as a screw compressor, having a rotor housing or casing 12 with a pair of overlapping bores 13 and 15 located therein. Female rotor 14 is located in bore 13 and male rotor 16 is located in bore 15. The bores 13 and 15 generally extend along parallel axes, A and B, respectively. In the illustrated embodiment, female rotor 14 has six lobes 14A separated by six flutes, while male rotor 16 has five lobes separated by five flutes. Accordingly, the rotational speed of rotor 16 will be 6/5 or 120% of that of rotor 14. Either the female rotor 14 or the male rotor 16 may be connected to a prime mover (not illustrated) and serve as the driving rotor. Other combinations of the number of female and male lands and grooves may also be used.

[0019] Referring now to FIGS. 2 and 3, rotor 14 has a shaft portion 23 with an end face 24 formed on the end of the rotor 14 radially outward of the shaft portion 23. Shaft portion 23 of rotor 14 is supported in outlet or discharge casing 53 by one, or more, bearing(s) 30. Similarly, rotor 16 has a shaft portion 25 with an end face 26 formed on the end of the rotor 16 radially outward of the shaft portion 26. Shaft portion 25 of rotor 16 is supported in outlet casing 53 by one, or more bearing(s) 31. Suction side shaft portions 27 and 29 of rotors 14 and 16, respectively, are supportingly received in rotor housing 12 by roller bearings 32 and 33, respectively.

[0020] In operation, for example as a refrigerant compressor, assuming male rotor 16 to be the driving rotor, rotor 16 rotates engaging rotor 14 and causing its rotation. The coaction of rotating rotors 16 and 14, disposed within the respective bores 13 and 15, draws refrigerant gas via suction inlet 18 into the grooves of rotors 16 and 14 which engage to trap and compress volumes of gas and deliver the hot compressed gas to discharge port 19. For the reasons discussed hereinbefore, it is necessary to maintain an end-running clearance 60 between the end faces 24 and 26 at the discharge ends of the rotors 14 and 16, respectively, and the facing surface 51 of the end plate 55 of outlet casing 53. This end running clearance 60 is defined as the region between the closest interface surfaces of the rotor end faces 24 and 26 and the facing surface 51 of the end plate 55. This end running clearance 60 establishes a potential gas leakage path, both circumferential and radial, between rotor end faces 24 and 26 and the end plate 55 of the outlet casing 53. As in conventional oil-flooded compressors, lubrication oil that naturally flows into the end-running clearance 60 serves as a seal to reduce gas leakage through the end-running clearance.

[0021] In accordance with the present invention, the interface surface area defining the end running clearance 60 between the rotor end faces 24 and 26 and the facing surface 51 of the end plate 55 of the outlet casing 53 is reduced by removing material from the rotor end faces 24 and 26 or the facing surface 51 of the end plate 55. As the interface area for a given end running clearance is reduced, the friction losses caused by viscous forces due to the oil in the region between the rotor end faces 24 and 26 of the respective rotors and the facing surface 51 of the end plate 51 of the outlet casing 53 is reduced.

[0022] In the embodiment of the present invention illustrated in FIG. 4, discrete, depressions 44, 44A and 46, 46A are formed in the rotor end faces 24 and 26, respectively. Depression 44 is provided in the central region of the end face 24 of the female rotor 14 to extend about the shaft 23 and depressions 44A are formed in the lobes 14A. Similarly, depression 46 is provided in the central region of the end face 26 of the male rotor 16 to extend about the shaft 25 and depressions 46A are formed in the lobes 16A. The shoulders 45 separate the respective depressions 44 and 44A in the rotor end face 24 and the shoulders 47 separate the respective depressions 46 and 46A in the rotor end face 26. Thus, the depressions 44, 44A, 46 and 46A comprise discrete, unconnected depressions and the interface surface area is reduced to the surface area of the shoulders 45 and 47. The depth of the depressions 44, 44A, 46 and 46A, although not critical to the invention, advantageously lies in the range from about 0.002 inch to about 0.50 inch.

[0023] In another embodiment of the present invention illustrated in FIG. 5, discrete depressions 54 and 56 are formed in the rotor end faces 24 and 26, respectively, in a petal-like pattern. As in the previous embodiment, each of the plurality of depressions 54 and 56 are discrete, unconnected depressions separated by shoulder portions 55 and 57, respectively, and the interface surface area is reduced to the surface area of the shoulder portions. The depth of the depressions 54 and 56, although not critical to the invention, again advantageously lies in the range from about 0.002 inch to about 0.50 inch.

[0024] In a further embodiment of the present invention as illustrated in FIG. 6, the surface of each of the rotor end faces 24 and 26 comprises a textured surface having a plurality of small depressions 64 formed between rises 66 and dispersed extensively across substantially the entire surface of the rotor end faces. The interface surface area is reduced to the surface area of the rises rather than the surface area of the overall rotor end face. Again, the depth of the depressions 64, are not critical to the invention, but advantageously have a depth in the range from about 0.001 inch to about 0.20 inch.

[0025] After studying the embodiments described hereinbefore and illustrated in the drawings, one skilled in the art will recognize modifications in the described embodiments. For example, the interface area between the rotor end faces 24 and 26 and the facing surface 51 of the outlet casing 53 may be reduced in accordance with the present invention by providing depressions in or a textured surface on the facing surface 51.

[0026] Although the present invention has been specifically illustrated and described in terms of a twin rotor screw machine, it is applicable to screw machines employing three, or more rotors. Therefore, the present invention is intended to be limited only by the scope of the appended claims. 

What is claimed is:
 1. A screw machine comprising a housing defining at least one pair of parallel, overlapping bores, an outlet casing having a facing surface, and a conjugate pair of intermeshing rotors located in said at least one pair of bores, each of said rotors having an end face, said end faces of said rotors being spaced from said facing surface of the outlet casing and defining therewith an interface area; characterized by the interface area being reduced by reason of a reduction in the surface area of at least either said rotor end faces or said facing surface of the outlet casing.
 2. The screw machine of claim 1 wherein at least one depression is formed in said rotor end faces.
 3. The screw machine of claim 1 wherein a plurality of discrete, unconnected depressions are formed in each of said rotor end faces.
 4. The screw machine of claim 1 wherein said rotor end faces comprise textured surfaces. 